1. 22.2 Important Coenzymes in Metabolic Pathways
Coenzyme A (CoA) is made up of several components: pantothenic acid (vitamin B5), phosphorylated ADP, and aminoethanethiol.
Learning Goal Describe the components and functions of the coenzymes NAD+, NADP+, FAD, and coenzyme A.

2. Metabolic Reactions
Metabolic reactions that extract energy from food involve both oxidation and reduction reactions.
Oxidation involves the loss of hydrogen or electrons by a substance or an increase in the number of bonds to oxygen.
Reduction is the gain of hydrogen ions and electrons or a decrease in the number of bonds to oxygen.
In both types of reactions, coenzymes are required to carry the hydrogen ions and electrons from or to the reacting substrate.

3. Metabolic Reactions
A coenzyme that gains hydrogen ions and electrons is reduced, whereas a coenzyme that loses hydrogen ions and electrons to a substrate is oxidized.
Core Chemistry Skill Identifying Important Coenzymes in Metabolism

4. Structure of Coenzyme NAD+
NAD+, nicotinamide adenine dinucleotide,
is an important coenzyme in which the vitamin niacin provides the nicotinamide group, which is bonded to ribose and the nucleotide adenosine diphosphate (ADP).
is required in dehydrogenation reactions that produce carbon–oxygen double bonds, such as the oxidation of alcohols to aldehydes and ketones.

5. Coenzyme NAD+
The oxidized form of NAD+ undergoes reduction when a carbon atom in the nicotinamide ring reacts with 2H (two hydrogen ions and two electrons), leaving one H+.

6. Coenzyme NADP+ NADP+, nicotinamide adenine dinucleotide phosphate,
is used in anabolic reactions, such as lipid and nucleic acid synthesis.
is similar to NAD+ except that a 2′ OH group is replaced by a phosphate group.
is reduced to form NADPH.

7. Structure, Coenzyme FAD
FAD, flavin adenine dinucleotide,
contains ADP and riboflavin (vitamin B2).
undergoes reduction when the two nitrogen atoms in the flavin part of the FAD coenzyme react with two hydrogen atoms (2H e−), reducing it to FADH2.

8. Structure of Coenzyme FAD
The coenzyme FAD (flavin adenine dinucleotide) made from riboflavin (vitamin B2) and adenosine diphosphate is reduced to FADH2 by adding two hydrogen atoms.

9. Function, Coenzyme FAD FAD, flavin adenine dinucleotide,
participates in reactions that produce a carbon-carbon double bond.
is reduced to FADH2 with the aide of enzyme succinate dehydrogenase.

10. Structure of Coenzyme A
Coenzyme A is derived from a phosphorylated ADP and pantothenic acid bonded by an amide bond to aminoethanethiol, which contains the —SH reactive part of the molecule.

11. Function, Coenzyme A Important functions of coenzyme A include
preparation of small acyl groups such as acetyl for reactions with enzymes.
production of the energy-rich thioester acetyl CoA.

12. Types of Metabolic Reactions
Metabolic reactions, which take place at body temperature and physiological pH, require enzymes and often coenzymes.

13. Study Check Match the following terms to their description.
1) NAD+ 2) FAD 3) NADH + H+
4) FADH2 5) coenzyme A
A. coenzyme used in oxidation of carbon-oxygen bonds
B. reduced form of flavin adenine dinucleotide
C. used to transfer acetyl groups
D. contains riboflavin
E. the coenzyme after C O bond formation

14. Solution Match the following terms to their description.
1) NAD+ 2) FAD 3) NADH + H+
4) FADH2 5) coenzyme A
A. coenzyme used in oxidation of carbon-oxygen bonds 1) NAD+
B. reduced form of flavin adenine dinucleotide 4) FADH2
C. used to transfer acetyl groups 5) coenzyme A
D. contains riboflavin 2) FAD, 4) FADH2
E. the coenzyme after C O bond formation 3) NADH + H+